The present invention relates to a charging member and a charging device equipped therewith, the charging member being designed to charge a latent image supporting body such as a photosensitive body thereon used for the electrostatic latent image process in copying machines, printers, and the like.
The conventional electrophotoprocess for copying machines and printers consists of uniformly charging the surface of a photosensitive body, exposing this photosensitive body to an image through an optical system, thereby causing the exposed part to lose charge and form a latent image, supplying a toner so as to form a toner image, and transferring the toner image to a recording medium such as paper, thereby completing printing.
A common way to charge the photosensitive body has been by corona discharge. This charging system, however, needs a high voltage (6-10 kV) and hence is undesirable for machine safety and maintenance. Moreover, corona discharge evolves ozone which poses an environmental problem.
Efforts have been made to contrive new charging systems which work at a lower voltage than corona discharge without giving off ozone. An example is that of contact type which is designed to press a charging member with a voltage applied to it against the photosensitive body to be charged.
The charging member for this system consists of an elastic layer of rubber or urethane foam, for example and a coating layer formed thereon from a resin solution of nylon, acryl, or urethane, for example by dipping or spraying.
This charging system, however, poses a problem with the durability of the charging member which is in contact with the photosensitive body to be charged at all times. This problem does not exist in the corona discharge system that employs a wire for discharge which is not in contact with the photosensitive body.
In the contact charging system, the durability of the charging member is greatly affected by residual toner particles. In other words, a very small amount of toner remaining on the surface of the photosensitive drum because of incomplete transfer or cleaning sticks to or even penetrates into the surface of the charging member, which results in inadequate charging. The drastic measure against this trouble is to improve the efficiency of transfer and cleaning, thereby preventing residual toner particles from moving from the surface of the photosensitive drum to the charging member. With the existing technology, it is difficult to eliminate residual toner particles completely. One way proposed so far to prevent residual toner particles from sticking to the surface of the charging member is to coat the surface of the charging member with a fluoro resin or silicone resin having good stain resistance. Such coating is effective but is not sufficiently durable.
Residual toner particles have an adverse effect only a little on the image even though they stick in small amounts to the surface of the charging member. However, after repeated printing over a long period of time, residual toner particles sticking to the surface of the charging member penetrate into the surface of the charging member. Once this occurs, residual toner particles begin to accumulate with an increasing speed, resulting in apparently defective images.
The present invention has been completed in view of the foregoing. It is an object of the present invention to provide a new charging member and a charging device equipped therewith, the charging member is durable enough to exhibit stable performance over a long period of time without deterioration by sticking toner particles when used for the electrostatic latent image process for copying machines and printers.
In order to achieve the above-mentioned object, the present inventors have carried out a series of researches on the electrophotoprocess which involves the steps of uniformly charging a surface of a latent image supporting body, exposing the charged latent image supporting body to an image, thereby causing the exposed part to form a latent image, and supplying a toner so as to make the electrostatic latent image visible, with the charging being accomplished by bringing a charging member into contact with the latent image supporting body. As a result, it has been found that the charging member provides stable charge over a long period of time without being deteriorated by sticking toner particles, if it has a surface whose universal hardness is no higher than 10 N/mm2 at a depth equivalent to the diameter of toner particles or at a depth of 7 xcexcm. The present invention is based on this finding.
The universal hardness represents hardness at a desired depth in an extremely thin surface of the charging member. This depth is equal to the depth of the indentation produced by a toner particle penetrating into the surface of the charging member. In other words, this depth is equal to the diameter of a toner particle. This implies that if the surface of the charging member has an adequate universal hardness at a depth equal to the diameter of a toner particle, then it would effectively prevent the penetration of toner particles into it. Even though the diameter of a toner particle is not known exactly, it is practical to establish an adequate universal hardness at a depth of 7 xcexcm which is equivalent to the diameter of particles of common toners. The present inventors have found that a universal hardness no higher than 10 N/mm2 is adequate. The present invention is based on this finding.
The first aspect of the present invention resides in a charging member to be used in the electrophotoprocess which involves the steps of uniformly charging the surface of the latent image supporting body, exposing the charged latent image supporting body to an image, thereby causing the exposed part to form a latent image, and supplying a toner so as to make the electrostatic latent image visible, with the charging being accomplished by bringing the charging member into contact with the latent image supporting body, wherein (1) the charging member is characterized by having a surface whose universal hardness is no higher than 10 N/mm2 at a depth equivalent to the diameter of toner particles or (2) the charging member is characterized by having a surface whose universal hardness is no higher than 10 N/mm2 at a depth of 7 xcexcm.
The second aspect of the present invention resides in a charging device to be used in the electrophotoprocess which involves the steps of uniformly charging the surface of the latent image supporting body, exposing the charged latent image supporting body to an image, thereby causing the exposed part to form a latent image, and supplying a toner so as to make the electrostatic latent image visible, with the charging being accomplished by bringing a charging member into contact with the latent image supporting body, wherein the charging device is characterized by being equipped with either of the charging members (1) and (2) defined above.